As Scooter said, don't stretch the regs.

Obviously if you buy a block of 20 hours you both can't log exactly 20 hours. The safety pilot cannot legally log run-ups, take-offs, and landings. So maybe you each end up with about 18-19 hours. Now would the FAA or anyone else find out if you each logged the full 20 hours? Probably not unless something happened to you down the road and they went scouring your logbook to verify times, but if that were to happen I'm sure you'd end up pretty screwed. Probably not worth it for the extra hour or two. Try to make it as 'quality' time as possible. Fly practice approaches and make the best out of the time under the hood.

As for the splitting of the costs, if you were a private pilot taking a friend up for a ride you could charge them their pro rata share of rental/fuel expenses. I don't see why it would be different for a 'friend' you are taking up as a safety pilot, especially if you are switching back and forth between PIC at the controls and safety pilot roles. I wouldn't think two people going flying together and spitting the costs evenly would be considered running a commercial operation that requires a commercial operating certificate since private pilots are allowed to do it. According to that reg, you could technically take a couple buddies in the back seat and all go down to Florida on spring break, and have them help pay up to a quarter of the rental expenses.

There is always the issue of whether a potential employer will except safety pilot time, but that isn't really relevant here. In the event of an accident, you could also run into some insurance problems if the person doing the flying isn't checked out in the aircraft, doesn't meet the insurance requirements, or doesn't have required renter's insurance.