How do I fix it? The wing joiner is stuck in both wings...

What kind of plane?

If it's an ARF, the best solution would probably be to order a new wing set. To try and fix this is going to involve some heavy surgery I'm guessing. And may not be worthwhile or possible to save.

Its an avistar, this is the second time its happened. I fixed it the first time, it took ALOT of nail polish remover took get them out. I have another wing but it has different colors on it.

I had a similar experience while learning to fly, I broke the wings on my trainer (royal air 40-t). I used a dremel tool with a cut off wheel to cut the joiner out as far as it would reach, then I just scraped away at it with a knife, and saw blade...took a while but it came out.

After making a new joiner I glued repaired and glued the wings together. You may want to add fiberglass cloth to the joint to further strengthen it.

Are u relying on JUST the wing joiner for joining the wing??
you should be gluing the 2 wing halfs together with epoxt all the way along the mating surfaces AND if practical wraping the join with a badage.

As Woodsy says above. Also use a heavier joiner than what comes. Make sure it is not light ply but regular birch ply. You coulddo what I did once with an Eagle II ARF. Add a joiner behind the spar. Cut the covering a bit. Open up a bit of sheeting if required and if you can glue another joiner in behind and against the spar, good. Then do another in front. Not the prettiest, but if you do it under the wing and cover it, it'll look OK.

When it's all joined glass the centre top and bottom and then finish the covering.