Um, NO! Sorry, but the other guys are wrong on this. The regs don't clearly say that you can't log dual whenever you want, but the FAA has interpreted that the logging of dual is intended for actual training purposes, such as:

- Training towards a certificate/rating/endorsement
- Currency (BFR/IPC)
- Aircraft FAM flight
- Area/route FAM flight

The key to all of these is that there is reasonable ending point...ie eventually the student is trained and no more dual is required. You could probably get away with giving lots of dual to a pilot who needs a babysitter due to insurance or even personal safety concerns, but the big red flag here is two cfi's giving EACH OTHER repetive dual on an ongoing basis. No legit training is occuring here...if you're qualified to GIVE dual on the first leg, you obviously don't need dual-received on the return leg!

Safety pilot is the way to do shared time-building. Legally this is perfectly acceptable, although some airlines don't give you full credit for SP time or dual-received.