just buy 2 'belkin' brand 25 pin printer extension cables ( female to male)
cut them in half, strip all the wires, crimp quick connects onto the male end of one side of your cable(on the part you stripped) connect those quick connects to your arcade controls.
now take the female end of one of your cables, strip all the wires, (paying close attention to what color wire is attached to which button/direction on the arcade controls) connect the corresponding colored wire from your female end into your keyboard encoder,
now plug your male end (control panel) into the female end (key encoder), at this point test yoru controls everythign should work like before, if it does, you are ready for the next step.
take another female end of your cable, strip all those wires and connect them to yoru hacked dreamcast pad, ( im assuming you got a hacked pad from some one as you said you dont solder, if not, hack your dc pad with wire, connect the wires to a barrier strip, wire the 25 pin cable to the barrier strip, this saves frustration later if you have to switch something or your cable gets messed up)
now when you want to switch, you unplug the cp from one female connector and plug it into the other, so make sure you mark your female ends for quick switching,
and for all dreamcast hacks, each controller must have its own ground, so you have to have 2 daisy chains of ground wires, they can both goto the same place on the key encoder, that doesnt matter, but for the dc it does, each player has its own.
this is how i run all 3 switchable control panels and a dreamcast in one cab. you will need to be able to switch your vga cable from your pc to the dc also, so you either need a pass thru vga box for the dc or unplug the cable and switch it or get a kvm switch, i just unlug the cable and switch it but that is unelegent.
good luck, holler if you need more guidance