I have 3 EZ-Pass transponders!
1 for 2 axels 2L(truck only) truck is under 7' tall, if over H
1 for 3 axels 3L(truck & single axle trailer)
1 for 4 axels 4L(truck & tandem axle trailer)
I did use only a 2 axle tag and pulled a small utility trailed occasionally, no problems.
Then I got a TC and got charged the maximum rate, because the cab over blocked the tag and didn't get recorded. $50 (4H) for traveling form Buffalo to Poughkeepsie!
Got that straighten out an then they told me I needed 3 tags, sent them to me no charge with shielded bags to keep them in.
This is for NY (www.ezpassny.com), go online google EZ-Pass state for specific details.