NOTHING whatsoever is too fishy not to have happened.
This is a definite messy situation.
I sorted one out a few years ago; not only did the shipper load on different trucks but used different carriers!
Man...was THAT ever a screw-up!
Anyways; the basic rule is that the papers presented to clear a shipment...whether a one piece one pound or a multi piece truck load...must only account for the goods being cleared at that time.
The shipper should have created two sets of papers; one for each truck. THAT is the easy way to handle the scenario. Unfortunately; shippers rarely take the easy way out.
One way to handle a shipment loaded on two trucks is for BOTH TRUCKS to be presented to Customs at the same time. Both drivers go to the broker; get the entry; list both Cargo Control Numbers on a B3B form and get cleared that way.
This is another procedure; GOODS TO ARRIVE that can be followed as well.
If you find yourself in this situation; tell your dispatch that the load will be a problem. Have dispatch call the Broker to see the best way to handle the situation. Follow the instructions exactly and given.
There are no hard and fast rules; each situation is different. It depends on the paperwork, the client, the border crossing point etc.
ALWAYS...when accepting a load to Canada... make sure the shipper doesn't toss something else that might be laying on the dock and has to go. Tell them everything MUST be on the Customs papers. You ;the driver/carrier; can be fined for undeclared goods on your vehicle.
I hope this helps.