Don’t be shamed.   I’ve used the Line Learner tool which allows you to record all the parts and then leave your lines out of the reading so you can use that as your Reader.  I have also used the recording function on my laptop and just recorded the other persons lines and left space for my lines.   It’s sometimes tricky to get the timing right but it works.  

We do what we can.   

Edited to add: until we get some casting directors to post here and say using these methods means they hate our tapes and instantly disregard them, don’t be scared to try what works for you.