The openSPOT is a simplex device, and it reports this correctly to the server. The server should honor this and send only one timeslot, as in DMR simplex devices only use 1 timeslot.
The openSPOT forwards everything it receives from the server to the transceiver and vice versa. You can set the "DMO mode TDMA channel" setting on the Homebrew/MMDVM connector settings page (Advanced mode needs to be turned on to see this setting) which sets the timeslot number of packets sent to the server (TDMA channel 0 is TS1 and 1 is TS2).
With the new beta firmware you'll be able to use the TS_1 and TS_2 MMDVM protocol options, which you can use to set talkgroups for both timeslots on the server.