Mounting two speakers on a supergun may be a bit cumbersome and pointless too, as two speakers make sense to me only if they are far enough from each other. So I suggest you use 2 speakers only if you intend to put them externally (which is what I did with my own supergun).
If you're going to wire two speakers for mono, then you can do it either serially or in a parallel manner. Serial means:
from + on the jamma harness to + on speaker 1, then from - on speaker 1 to + on speaker 2 and from - on speaker 2 to - on the jamma harness. In this case the impedence must be 4 ohm for each speaker (total 8 ohm).
Parallel means:
from + on the jamma harness to + on speaker 1 and from there to + on speaker 2. Then, from - on the jamma harness to - on speaker 1 and from there to - on speaker 2. In this case the impedence must be 16 ohm for each speaker (total 8 ohm).
Also, you might want to check <a href="http://www.neo-geo.com/cgi-local/ultimatebb.cgi?ubb=get_topic&f=5&t=002513" target="_blank">this thread</a> for a project about switching between mono and stereo with the same speakers (I haven't tried it myself though).
