our bass player and drummer have never played together and really need to be in locked in tight with their playing - any suggestions on what kind of exercises they can drill through together before band rehearsal so they can lock in and provide a tight groove for the rest of the band?
Start with your basic 4/4 and 3/4 timing. Simple chord changes; play with the tempo a bit. Figure out a scheme so that you both know how to watch for and signal changes. Some bassists love the right side of the stage because it allows them to see the drummer's right foot; which is how he or she is going to keep the time (with the bass drum pedal).

Once you've got an easy but solid lock, then try some odd time signatures. Start slowly and keep up with the changes. It is a matter of playing together so that you each get to know the other's rhythm. And don't just follow the drummer or you'll get lost in the mix. Try to stand out from time to time, but still keep that lock strong.
"Maybe this world is another planet's hell?" - Aldous Huxley
Yeah I definitely suggest to start of easy. a simple kick, snare, hi hat groove on the drums, and a simple root and fifth notes on the bass. at least until they get comfortable with each other then they can start turning it up.
The best thing they can do is get used to playing with a click track or a metronome. For the bassist practice with a drum machine helps. If they can keep tempo there will be no problems locking in.

To be honest the only thing that will throw them if they are inexperienced is how to deal with the mistakes the rest of the band make which result in breaking tempo.