Both are contracted for next season to their respective teams. I think Perez earns around £10m, he's not going to willingly swap into a car that seemingly can't now get into the midfield after some early podiums. So they'd probably have to pay Perez off to make it happen.
What would be in that for Red Bull? They're still likely to end up with a 1-2 in the Driver's championship, they'd have to pay Perez off, pay Alonso and still end up with a 1-2, plus Alonso realistically only has a year or two left in F1 so it just makes no sense.
Would love to see Alonso @ RB. Max has been strolling the championship for the past few years, bar the odd race, Perez has barely given him any trouble and is obviously number 2 at the team.
Would be amazing to see Alonso give him a run for his money. It would also be a much better indication as to where Max truly sits in the GOAT race. I know you'd have to caveat that with; the car has been developed for Max and is therefore far more suited to him but if Alonso could get a few early season wins it would be hilarious watching RB fall over themselves trying to justify backing Max as the team's number 1.
All that said, if they brought him in as a support drive that would suck balls.