Gradual tempo increase of two different bpm tracks

[RPGWiZaRD]

I cannot for the life of me figure out how to do this in Reaper or if it's even possible. I'm trying to make a mix of a couple of tracks and at the end of the mix I have both a 150 bpm and 170 bpm track and I was thinking of mixing so the first 25 secs of the tracks are gradually speed up from 150 to 170 bpm so the 170 bpm track is slowed down to 150 bpm at start and then gradually during first 25 secs or so speeds up to 170 bpm and obviously same thing for the 150 bpm track. I've tried adjusted playback rate of the 170 bpm track to be 150 bpm and then using tempo markers to try speed up those two tracks from 150 to 170 bpm over a 25 sec duration but it just doesn't work for me, either the rate of the tracks are off or it doesn't speed it up gradually etc.

I guess I just don't understand Reaper when it comes to working with tempo, apart from that I enjoy using Reaper but it's very confusing to mix tracks with various tempo in Reaper, typically I just slow down the entire mix intro by 5 bpm and then speed it up instantly after the mix part but in case of a bigger step like 150 to 170 bpm I'd really prefer to make it gradual. Seems like odd to have to use another program to get this done.