I have the same... but it works:
A) A phone with TomTom and no SIM-cards in it.
B) My usual phone without TomTom plus SIM-Cards.
B) is setup for tethering and A) simply connected to the WLAN that is generated by B)... So you simply have to tell A) to connect to the WLAN from B)... like you usually connect to any WLAN...
But this has nothing todo with TomTom... it's the usual way how tethering works.