I just drove the AlCan from Alaska to WA and back. I can easily make it down within the 72 hours so I was hoping to go down, pick up my trailer and go north again within the 72 hours. Unfortunately for a lot of reasons I ran into all of the fires around Conconully and Lake Chelan.
Yes, you need tests that are no older than 72 hours each time you cross so you may need two or three tests.
I wouldn't run back and forth. The testing places can't guarantee when you will get the results back. It depends on the volume of their tests to work thru, the day of the week and time of day you take the test. The best thing to do is be at the testing place before they open, take the test, and then hope you get them within 4 - 5 hours. Some places will text you when the results are ready and others won't - check with them and be sure.
Remember - each time you cross, you will be dealing with a different Canadian Customs agent and they all do things differently and be assured the second agent will not do it the same as the first agent. Have copies of your shot and testing records for each crossing. Often, but not always, they will want to keep a copy of them SO HAVE EXTRAS.
If you're crossing the border every day or several times in a short time span, I'd plan in your time to be inspected / searched. They could suspect you're hauling things you shouldn't be, back and forth. And it could be US Customs will inspect you.
I'd say you're going to spend half of your time dealing with shot and customs issues. I'd go north and stay there, don't cross back and forth.
The ArriveCan app was a waste of time. You have to put everything in on your cell phone, you can't do it on a laptop or desktop. I downloaded the app, fought with the first app for a couple of hours, got most of the info in and then a message came up and told me to download the new app.
I downloaded the new app and it was no better, but I got through it. I hit the finish / enter button and a message came back telling the app was full, with no alternatives. I hopped in the truck and headed south.
I got to the border, where it was taking about 30 minutes per vehicle, and the agent asked if I had used the app. I told her yes, but it didn't work. Luckily I had saved the last page of the app stating it was full. She didn't like it, but asked another 100 questions and had me come inside to fill out some forms.
On the way north a couple of days later, with a new covid test, I wasn't asked about the app at all. One of the problems with the app is it was developed for people arriving by plane - it wants to know your exact time of arrival and at what airport. Do you think anyone driving can tell them the exact time - what if you sit in the customs line for a couple of hours. And driving you could enter thru a dozen different check points. Hopefully they're done with that app, but who knows, it's up to the particular agent you get.
Go north and stay there until you're done.
Bill