I have a very specific concept for a character in mind, effectively a sneaky assassin bard, so I thought college of whispers bard would fit this well and it does, but it still falls behind in damage quite a bit due to bard being a pretty support focused class overall. so I'm thinking of multiclassing into rogue (don't know what subclass yet, suggestions are welcome) for that boost in melee damage and sneakiness and what not. but I'm wary that multiclassing unless done right can often just result in you falling behind the rest of the party in terms of power, so I wanted to ask around if anyone's tried this before. Or if people better at the game than I am know if this combination would remain as useful to a party as going as a single class, specifically in terms combat.

 People have absolutely tried it before. A quick Google search will show you countless people trying to find the perfect blend.

Would it be useful? Yes. I don't think you'd fall behind. Your fellow adventurers are going to still enjoy the perks of your inspiration, the skills between the two classes have good synergy, and the spike damage from Sneak Attack is always great. I think the character would have great utility in and out of combat. I'd go for a high CHA & DEX build, in that order.

Maybe talk to your DM as well? To see how that character would fit into the setting. If it is a great fit then you are winning on multiple fronts.

thanks for the quick response, I was definitely thinking high CHA and DEX. I did do a quick search previously, but I figured I'd post something on here on the off chance any of it was outdated or something like that, just to be sure. and yeah I'll definitely talk with my DM before playing the character.

It partly depends on the mix you’re going for. The scaling of sneak attack means that a dip isn’t going to massively alter your damage output, and trying to transition to a new main class means you’re playing catch-up, and if you’re already 4+ levels in you’ll have a hard time pulling that off. Honestly, you might be better served looking at a feat like Fighting Initiate for the Dueling Fighting Style. 

mainly I'm just going for melee damage focus with some low level magic for utility. which, looking a bit further into it, an arcane trickster pure rogue might just be better. the main reason I wanted bard was primarily for flavour, but I could probably just get proficiency with an instrument and roleplay as a bard and I'd basically have what I'm looking for minus some low level bard features, as I was planning on going much heavier into rogue levels than bard levels anyway.

Yeah, that’s more in line with what you want.

Just a note on multiclassing. Usually there are three main drawbacks that people fall foul of:

1. The main one is competing options. Instead of getting options that stack, they get things that compete. So instead of being a level 2 character of being a Fighter and a Wizard, they end up having to choose between being a L1 Fighter or a L1 Wizard in each situation. Avoid that like the plague. You want to stack stuff or open up options (eg actually have a use for your Bonus Action) due to your multiclassing, rather than having lots of things to pick from because of it - the latter is a waste.
2. Becoming MAD. You want at most two ability scores that you're dependent upon. Don't go for Paladin Rogue's because then you have to try to level up your Str, Dex, Con, and your Cha, plus both Wis and Int would help. It's actually a similar theme to the above, you want to focus on doing a few things well, rather than many things in an average way.
3. Spell progression gets halted. You don't want to be relying on L1 spells when going up against a Tarrasque or a Lich. Often, having higher spells and more of the lower ones will serve you better than whatever abilities you gain from another class.

Of the three, only really the third is of any concern. Even then, it may not be an issue, since you're getting stuff that complements your character from the Rogue aspect anyway. 

If you think pure Rogue will suit your character better, crack on. As DM, if you weren't looking for mechanical benefits, I'd be quite happy for you to just have the ability to play an instrument for free. I'd also be willing for you to swap a proficiency for a proficiency in that instrument as well. Multiclassing wouldn't be a major problem either though, I think.

the only thing with going pure rogue as arcane trickster is that uses Int instead of Charisma, and whispers bard doesn't quite get the melee capability I'm looking for. and as you mentioned it runs to risk of falling behind on spell progression, but overall multiclassing does fit the character I have in my head best I think. thanks for the information 

I believe there are plenty of spell options for an Arcane Trickster that don't require a high DC, so you could still go primarily DEX/CHA. And even with sub-par CHA you can use Expertise to be better at CHA skills than anyone else in your party. 

If you want to focus on melee damage, every level of bard you take is going to hamper that. Bard is just not a damage-focused class by any means. Even something like Rogue/Warlock with a dip into Hexblade is going to be better for you.

yeah at this point I think I'm just gonna go arcane trickster and see how far I can get with a relatively low INT