Nope, you can still use both speakers.
The TIP connection on the PLUG goes to POSITIVE on the speaker. Then the SLEEVE goes to the the negative on the speaker. Then you can wire from the POSITIVE on one speaker to the POSITIVE on the other speaker, and then the NEGATIVE of one to the NEGATIVE of the other.
This is a parallel wiring scheme and will cut your overall impedance in half. So if you have 2 8Ω speakers, they would present to the amp as a 4Ω load.
Personally, I like the single jack option better. But after using a Two Stroke for a long time, I don’t disconnect one or the other speaker. Its pretty fab with both.