I have asked Sigma about whether they have a combo sensor to use on the back wheel so I can use them on a trainer, and this is their response:

> We do not make any combo speed/cadence sensors. Fortunately though, you can
> install both sensors on the rear chain stay and get speed and cadence off
> the rear of the bike. The speed transmitter will work from this distance to
> give you speed while on the trainer.
>
> Best regards,
>
> Chad Steward
>
> Customer Service Manager/
> North West Tech Rep
> Sigma Sport USA
> 3487 Swenson Ave.
> St. Charles, IL 60174
>
> 888-744-6277
>
> www.sigmasport.us

And then there was another follow up note sent to me saying:

"For better transmission, have the speed sensor on the top of the chain stay."

hope this helps