Time Machine backup with dual internal drives

Question re Time Machine on older two ssd capable MBP. I've searched a lot online and can't find an answer re how to backup a dual hard drive mid 2009 macbook pro (El Capitan 10.11.6 - I replaced optical drive with ssd) to a single external hard drive. There are a lot of instructions re backing up separate computers which seems to be something TM handles naturally because the external drive is physically inserted into 2 separate computers - TM knows the backup drive destination stays the same.

In my hardware setup I cannot find a way to make TM keep the same external backup drive location. Instead, it wants to use the 2nd internal drive as the sole backup destination (or use both) when it prompts me to select a disk. Perhaps it's not possible for dual drive Macs to do this. Any suggestions would be appreciated.