Yes, you can use your 2 line cordless with the Single Line Adaptor. The Single line adaptor for the Vodavi Starplus Flex gives you 2 analog exts. What I would suggest (since you only have 2 lines) is to connect the 1st analog ext into line 1 of your cordless and the 2nd analog ext into line 2 of your cordless. Then program the 1st analog extension to ring on line 1 only and the 2nd analog extension to ring on line 2 only, that way you will know exactly which line is ringing in on your cordless phones. Remember to install the the 1st port of the single line adaptor (KSU side) to a even number and the 2nd port (KSU side) to the next available odd extension number.
ie; ext 10 & 11 or 12 & 13 or 14 & 15 etc.
You could just use 1 of the ports on the SLA and connect to line 1 on your cordless and program that port to ring on lines 1 & 2 but you would not know which line is ringing and you could not get 2 calls at one time either.
Their is also a jumper inside the SLA that needs to be set to 308, 616 or 1224. It will be 616 for you.
Note: Just in case you do not know, you will not get caller ID on your cordless phones when connected to the SLA.
Hope this helps