The way I understand it (I could be wrong) - If two sources were unexpectedly tied together, power would likely flow between the two subs until both are carrying the same amount of load, but on the transmission level, it could be a bigger issue if one was on a branch line carrying a light load and the other on a heavily-loaded trunk line. You would then have power flowing from the heavily loaded line towards the lightly loaded line, likely taking the distribution circuit in between past its load-carrying limit.

As for the second question, that's a good question and likely comes down to the MVA capacity in the source sub and the load requirement of the respective subs.

To give you an idea as to the relationship between volts / watts / power. A circuit energized at 2400 volts (singlephase branch from a 4kV line) and carrying a load of 500 amps would equal a load of 1.2MVA. At 7200 volts, the same 1.2MVA load only requires 167 amps. At 40kV (phase to ground voltage from 69kV) that same 1.2MVA load now only requires 30 amperes.

Much thanks guys for the information! I had no idea so much went into designing power lines / the grid. The "Normal Open" schemes had me confused for quiet a while but this was very helpful in giving me some insight on how it functions. As for the 69kv question, that was sparked when I began noticing the differences between 69kv lines built by Xcel and the nearby G&T co-ops. Conductor size, number of subs served, insulators, etc all seemed a bit different between the companies and I was wondering the reasoning(s) for that.