I'm planning on playing a Swashbuckler/College of Swords Lightfoot Halfling in an upcoming campaign.  My buddy in our group plans on playing an Ancestral Guardian/Champion human variant.  I told him that I thought this was a great combo for battlefield tactics as I can act as a skirmishing support, retreating behind him after an attack and he can act as a tank all while we both can impose disadvantage against attackers targeting all other party members.  Our other party members that we know of so far are going to be blaster and support type characters(Ranger and Druid) as far as combat. 

How viable is this strategy?  Of course I know we need to know the other player's role, but based of this info, what can we sort of expect?  I haven't really got to play and have DMed for little one shots so I'm probably overthinking, but I'd like to see these characters through the campaign.

There are a couple scenarios and tactics that need to be thought about in regard to working out the effects of this tag team.

1st tactic is that it’s usually the most advantageous to take out a single creature completely by focusing actions on a designated creature, then moving on to the next one. Leaving multiple creatures with slightly less health isn’t nearly as effective as removing one or more of those creatures from combat. 
  In this tactic, the swashbuckling rogue may be of more use actually staying near the barbarian to make better use of their increased effective hitpoints. A barbarian giving disadvantage on attacks against the party members isn’t effective if the creature still continues to attack the barbarian as a normal die roll, or possibly with advantage due to reckless attack. 
  I’d say it’s the barbarian that needs to attack and back up, not the rogue. The barbarian subclass ability drastically increases the tankiness of the rogue against attacks from that creature, a rogue already has uncanny dodge, and the level 6 barbarian feature can further reduce the damage if an attack actually lands on the rogue. The rogue would also have resistance to all of the attacks that land against it from that creature.
  This is counter intuitive to the design of a swashbuckling rogues built in “mobile” features, but remember a rogue can choose when to sneak attack. A swashbuckler is free to make an attack against a particular creature that isn’t the target and not use sneak attack even if they hit, and then move to the creature the barbarian may have designated to try a second two weapon fighting attack to deliver sneak damage.

the 2nd scenario is if you happen to be fighting multiple creatures you actually think are a threat, but cannot focus fire  due to some sort of game limitations (environmental limitations or perhaps role play reasons). It doesn’t seem like it at first reading, but there actually isn’t a range limitations on the ancestral guardians disadvantage imposing feature. They are free to use reach weapons and even ranged weapons to impose disadvantage against a creature. So if a creature is particularly difficult to get to and is raining down attacks, the barbarian can actually still impose disadvantage as long as a single attack lands, even from a longbow. You can even use an artificers spell storing item to possibly use  a spell attack, as the ancestral protector only requires a hit with an attack.

I think it's a good combo. As a swashbuckler you can still sneak attack without an ally next to you, so having the barbarian standing 10 ft away with a reach weapon imposing disadvantage on your melee target while staying out of reach themselves is an interesting sweet spot with this party composition. If they attack you they have disadvantage from the barbarian, if they move to attack the barb, they get a small attack opportunity attack from you. Later on you can taunt them to exacerbate this and the barb can reduce any attacks that land on you, which you're already resistant to and can halve again with your reaction. 

The damage from that small attack of opportunity is actually quite substantial. A rogue is able to utilize its sneak attack die on a hit so long as sneak attack hasn’t been used in that turn. Since this movement choice is happening on the target creatures turn, sneak attack is fair game again even if it was used on the rogues own turn. This synergy increases offenses and defense in what seem to be mutually exclusive scenarios.

Rogue gets targeted, it has a disadvantage imposed on attacks against it, resistance if it hits, rogues reaction to halve damage, barbarians reaction to further reduce damage.

creature decides to ignore the rogue, goes after barbarian or some other creature, rogue gets opportunity attack that roughly increases its own DPR around 30-40% against the designated creature. A similar thing can be done if the rogue takes the sentinel feat. If the barbarian stands side by side with the rogue, and the target creature attacks the barbarian without moving, sentinel gives a reaction attack that may more reliably be used. This makes attacking either the rogue OR the barbarian a bad idea. Excellent coverage.

It really is a wonderful team.

That was meant to say "sneak" attack opportunity. Ahh, autocorrect.

Ohh. Yea I feel that. I interact with these forums exclusively on my phone. I make mistakes quite often because of autocorrect along with my lack of texting proficiency…