I just did something like this...It worked out all the way around as I needed to split the ticket for billing purposes anyway.

Ticket #1
MYR-ATL 5/22 a.m.
ATL-MYR 5/27 p.m.
$279.50

Ticket #2:
ATL-PIT 5/24
PIT-ATL 5/27 a.m.
$178.00

Total is $457.50, but if I'd booked a straight 'Triangle:' MYR-ATL-PIT-MYR, it priced at around $770!

When I called for upgrades, the agent said it was perfectly legal, upgraded me on all in the time window, Q'd me for the rest, and thanked me for my business.

So I get 6 segments, not 5 for less money, and my only risk is getting to ATL on time, but hey, ya gotta take a chance once in a while, and US is all at the end of "D" anyway.