Welcome to Rig-Talk.
Your thread might be better served in the main forum, this is more for forum Feedback. Lots of smart people here so I'm sure you'll find someone to help. To get things kicked off I'll start with:
? What is the Ohm output of each cab - has inputs for 4 and 8
? Can you take the back off the cabs and get speaker type
? Are the cabs identical - yes
? How do you like the tone of the Mini 25w - I'm still trying to get it dialed in
? Are you comfortable in changing settings on the RP500 - plenty left to learn
! I had an RP250 - not a bad unit, I wouldn't worry about effects yet
? Can you buy an ABY pedal so you can play both amps at the same time - yes, figuring out I need one, any recommendations?
! Sell the Spider and RP500 and get a used AxFx FX floor model down the road
! Budget depending I would look at a Mesa 212 with V30 speakers or maybe an Avatar 212 with your choice of speakers - thanks!