So lets say a player wants to create an Item (Daemon weapon, for example) and it says it will take two years to complete.

How do you incorporate that where the players don't have to put everything on hold for two years? Or so while he is creating that, the other player won't be forced to stop the adventure for two years?

Well...

whenever a player wants to undertake some construction (like the daemon weapon example above) she should ask herself (or the GM) if the construction time is within the scope of the game/campaign currently played.

The possible answers are

- yes
- sorry, but no
- dude, I don´t know because I only move from one bought module/freebee online adventure to the next here!

If the downtime accumulates to enough time for the task << very well
If it did not << "well, I tried and it made for a nice background thing of my character"

If option two is not an option since "I WANT A DAEMON WEAPON!!!!" one should make it an ingame quest.

Have your player go to a place where time moves much quicker for them than the rest of the party, boom weapon done, and minimum of waiting for the rest.

I let my team have a retinue, one of which is a disciple from the silver forge on Qsal (topple spires adventure). She does have the weeks to spend crafting an item for team.

Also don't forget that space travel can take as long or short as you want. Months can fly by that way.

Have your player go to a place where time moves much quicker for them than the rest of the party, boom weapon done, and minimum of waiting for the rest.

You mean the warp?

The decision which needs to be reached with the player and party is whether his quest for a daemonic weapon is more important than their overall goal. If the party agrees then that can start a new questline, gathering the resources for it etc. If not then they have to make a choice. If they would go it alone or leave it then unless you want to run with them individually then they would move out of scope with the rest of the party and must retire/withdraw their character.

Then when its all together you could just do a montage/batman style spin and boom, two years later.