Well - obviously the $140 UA fare would require me to stay at least one day (since it IS more expensive if I book the 2 trips separately).
However, since these are two separate companies, I don't see how any minimum stay requirement could be imposed. I looked at COC and MP Rules, and there is nothing. I called UA and they looked into it, and all they found was rules applicable to UA ticket stock. They said it would be ok to book it... We shall see