Look at pg. 30 of the BGB, under the heading Blast Weapons.
My first question is were you guys using a blast marker? If so, why the argument? Either it completely covered one Obliterator base and partially covered the other one or it didn't. From the sound of your description, it sounds like it was a BS call, but any argument should have been avoided by the use of a blast marker.
Secondly, as a heads up, on the same page it has the rules for Multiple Blasts - each is resolved seperately. So, the process should have been:
1) Roll to hit.
2) Place marker, if a hit was scored.
3) Determine actual number of hits due to full/partial coverage of the blast marker.
4) Roll to wound/saves.
5) Go back to step 1 for the next shot until there are no more blast markers coming from that unit.
KW