Assuming the senders and the gauges are matched then the 2 nuts vs 1 nut should not have any effect.
Personally I would use 2 nuts like your first photo. Less chance of damaging the sender.
As for the trans temp...
Where are you measuring the temp?
It is not too surprising that there is no reading for about a mile. Your engine creates heat from the first start
The trans has to generate heat from torque converter slippage etc.
If the trans is plumbed through the cooler in the rad then the engine heat helps to warm up the trans.
As for peaking at 115, maybe that's it's operating temp under the conditions you drove it in.
If both gauges are by the same manufacturer, you could temporarily jumper the feed from the engine sender to the trans guage to be sure it reads above 115.