Sounds like a tuneing issue, I dont know how many tandems & single bikes I have repaired or tuned were bike shops and others have tried and tried at no end to tune or fix a problem just greating more issues. With that being said your situation may be special not seeing the tandem in person and knowing all the facts, condition of componets and tweaks maid. The componets should work fine if tuned right and set to spec accross the board. Tandems are a little more finicky and a little less forgiving than there single counterparts in the same configuration but if the parts are in spec and of proper condition all should work without tweaks like twisting the deraileur around and stuff like that. Good componets, good chain, proper tuning all should work.

Grip shifters only have three positions for the front Deraileur Low, Mid, and High. The Mid should allow you the use of all your rear cassette gears without scraping and relys on cable tension to hold this adjustment, to little it will scrape in the higher gears in the back on the middle ring up front, to much tension and it will scrape in the lower gears in the back on the middle ring up front. The Low or little ring up front should allow the use of 4 to 5 of your low cogs without an issue and the High ring or big ring up front should do the same on the other end of the spectrum. There should be enough adjustment on the front derailuer for the deraileur to be in the right position and allow several mm clearance in the small ring up front and the biggest cog in the back between the deraileur and the chain and the same goes for the big ring up front along with the smallest cog in the back ( there is a proper procedure for doing this adjustment on this type of set up with grip shifters, a lot of mountain bikes have this same issue only having 3 set points in the shifter Low, Middle and High or 1,2,3) There are other things to look at as well proper deraileur position is key along with low and high set points and proper cable tention as mentioned above.

Also as mentioned by others a properly set up Jump Stop is worth it's weight in Gold.

Best of luck,
Bill G