Yes,, but I believe in order for it to work the phone must also register as an extension on domain 2, you then just specify the line number for the BLF.
Here is a screenshot of my setup for a Yealink phone, I have line 1 as ext 201 in domain 1 and line 4 as ext 206 in domain 2:
I have many phone that have accounts set up in multiple domains, I provision any given phone from a device record in one domain only. the only slight issue we sometimes see is if I have, for example, BLFs for two extensions both the same number but in different domains, FusionPBX will sometimes confuse the labels in the device record when editing the extension records. My work around is to always edit the device record last.
I hope that helps.