If you're running LineageOS 17, I don't think there is a way to get TWRP functioning at the same time.
If you're running LineageOS 16, then TWRP works when using EX kernel. However, there is standby battery drain on LineageOS 16 with EX kernel on the Pixel 3a (device only deep sleeps around 60% of the time). I don't know if the battery drain occurs with the included kernel, but I suspect it does. This cuts standby time in half according to my testing.
Thank you xdadevnube. I'm on stock 10 rooted with Magisk, but missing TWRP. I'm considering going back to 9 with EX kernel so that I can get TWRP permanently. However, I'm also considering flashing GrapheneOS because I like its security focus, but the latest zips are based on 10. The main reason I want TWRP is so that I can take a system image for backup. If I can do that with ADB or another application then I don't need TWRP. Do you know of any alternatives that can take a system image of my phone? I have searched the forums unsuccessfully.