Okay, not too sure if this is the right place to post, but I need some help from DMs. I'm not a virgin DM but I've mostly enjoyed being a player.

I'm currently setting up to run two games a week for two separate groups.

One group are completely new and I'll be running Dragon of Icespire and the following adventures (I've built them into a coherent story) , while my second group are more experienced and I'm going to running LMoP into Tyranny of Dragons for them.

Both groups know each other and I've been fumbling around with the idea of having a crossover between the two once they finish their first adventures, coming out of them around the same level.

My idea is that the Icespire game will be set <insert amount of time> before Lost Mines. I'm going to replace the Sleeping Giant with a tower, it's one of the few buildings to remain whole after the Orc raids of 951DR that wiped out Phandalin and is now the home of a young, flamboyant wizard with a singular purpose: to see further into the future than anyone before him. I have a few ideas for quests, find a large mirror never touched by magic, find spell ingredients, assist in the spell. I'll spread these out over the quests that are on the Townmaster board already. After the party kick the crap out of Cryovain and save the day, a celebration will be held in Stonehill Inn and after some hand kissing, baby shaking and drinking - a guard will rush in and proclaim that strange light is coming from the tower. Our heroes will (hopefully) rush to see whats going on, some fluff from yours truly and x amount of the players look into the mirror to see the other party looking back at them. In his excitement, the wizard will falter in the casting of the spell, a crack will appear and a blinding light. The players who were looking into the mirror are replaced by some of the others. 

In the "future" game of Lost Mines, the tower is simply replacing the part of the story that would be Sleeping Giant tavern, however, now it's a damaged tower and the Redbrands are just using it the same as they would normally. After they clear it out the townmaster sets some people to task of clearing away rubble to discover a basement door leading to a storeroom and they go looking around. The party then carry on the adventure to slap everyone that they need to and become Champions of Phandalin. Same as above, celebration but they're taken down and find a kind of time distortion. I'll add the same wizard from the other game into this one, but much older and suffering some kind of amnesia like symptoms, calling the party by the other parties names and generally being in the way some of the time. The same crack appears and blinding light.

Now, getting the parties back to their own times. I want to run the same dungeon for the parties but it be slightly different due the passage of time, and would love advice on the general idea that I've had.