Let's say you have two swap disks, set up like this:
swap1: priority = -1 swap2: priority = -2
Is this going to be any different from the following setup?
swap1: priority = 47 swap2: priority = 10
No it does not matter how much the priorities differ.
But anyhow, priorizing swap areas is only usefull if they are of different speed. Since kernel 2.6 even swap files are as fast as a swap partition. If you set same priorities to swap partitions the kernel stripes pages similar to raid 0 resulting in better swap performance.
Usually it is not necessary to mess with the swap as the linux kernel does a good job in managing it.