How often do you replace the timing chain?
When it's showing about 75% wear on a Park chain checker.
Do you buy one chain from a shop or just combine two at home?
You can usually get 2 timing chains out of 3 normal chains... combined at home.
Is there any special do's and don'ts for combining?
If you want to be politically correct, use the same brand / model chain and the recommended one-time-use rivet and/or a re-useable chain link from SRAM, SuperLink, etc. You can't remove the sync chain with a re-useable link, but it does make it easier to untangle during cleaning, etc.
As for the same brand / model, it really shouldn't matter so long as they are the same width... but that's where it can get interesting since Shimano, SRAM, Campy and KMC all seem to use slightly different width chains for their respective 8, 9 and 10 speed models.
What brand and model do you suggest for replacement?
Whatever you can find on sale that suits your fancy. Timing chains are low-tech and don't depend on special ramping, etc to work.