Efficient? It should be workable.
The two tapes should stay in sync, for the most part.
Cassette's a budget format to begin with, and 8-tracks on cassette is a small modern miracle. Syncing two cassette 8-tracks should be fine, with all the limitations and considerations you'd have with any sync'd set of units. If there's any mechanical slippage in either 688 cassette mech, you could be subject to the tracks drifing apart, but in theory if all things are mechanically sound, it should work fine.
A single #-track platform will always be a little more stable and robust than most of the sync systems out there, but a sync system should be functional.
Something like an MSR-16 would outperform two 688's in every way, with the added expense, size, added full sized mixer, etc. Syncing two 688's is feasible, if you want to go that way, but it will obviously never perform like a larger system.
With that being said,... DO IT!
