Ya, that's how it works in real life. The only difference is that you don't have to be "precise" about it under the current rules. Technically two blocks of C-4 a foot from each other magically is going to do much more than one block twice as big. Personally I'd force a Demolitions check and count it as one explosion if they failed (and of course signficatn failure could also mean that they don't detonate at the same time).