Bill wrote:
For safety of anyone working on these outlets in the future, I would suggest
using a tiebar breaker (like for 240 volt circuits) for the circuits in
these boxes and placing a note in the breaker panel that these outlets are
on two circuits.

The tiebar will force them to turn off both circuits at the same time.

This would keep someone from turning off the circuit to say the left outlet
and still having live power on the right side and zapping themselves...

I'm sorry, but IMHO, anyone who doesn't check all wires in a box for
voltage deserves what they get......as they don't have any business
doing electrical work in the first place.

Here's the preferred tool to use:
http://www.emsco-usa.com/fluke/flvolt.htm

IMO, using a tied breaker just sets up an unqualified person who has a
very bad habit of not testing for a presence of voltage on all wires
for a wake up call. It doesn't happen a lot but sometimes circuits are
accidently backfed. If the backfeed is on the same leg, the miswiring
often goes unnoticed. I saw this on a circuit just two months ago on a
service call. A tied breaker won't protect against it, as the circuit
will still be fed from another circuit. Bottom line, the first rule to
doing electrical work is to TEST ALL WIRES, even equipment grounding
wires, for voltage with a known to be good tester. If one doesn't want
to invest the $25(US) for the Fluke Volt Alert, or similar device, the
old standby $3 neon tester will do, although more time consuming. And,
yes, both can sometimes give a false positive from induced voltages, in
which case one should investigate further with an analog meter or a
test lamp.