Can you text between 2 iPhones internationally (US to Canada) for free?

No. Texts are part of your phone company/carrier tarrif, so you are charged for them in the same way as you are charged for calls and data usage as part of your tarrif. Normally, you will have tariffs that offer a certain number of minutes and then perhaps unlimited texts and then with a certain amount of data. This will all be included for a monthly fee, so it is not free. If you go above your allowance, you will be charged more by your phone company, but no, it is not free because it is all part of the fee you pay monthly to your phone company.

Nothing is for free.

You can iMessage between two iPhones internationally for free, but not with text messages as in SMS/MMS.

Exchanging iMessages is similar to exchanging email - internet access is required via an available wi-fi network or via your carrier's cellular network.

iMessages are not text messages as in SMS/MMS. SMS/MMS are exchanged via your carrier's cellular network only which is not free.

You're welcome.

On your iPhone go to Settings > Messages > Send as SMS > Off.

At the same location, turn MMS Messaging off.

This will not prevent you from sending an SMS to a non-iPhone cell phone user. If the send button for the message is green, the recipient is not an iPhone user.

Well, if you don't send or receive any SMS/MMS on your phone whilst you are away you won't be charged anyway, so I don't quite see the point of that.

If you want to use iMessage, then you have to turn it on under settings and then use Wifii.

Both iPhones need to have an icloud account iMessage enabled and be connected to a Wifi network.

When you do that the message bubbles change colour to Aqua Blue and are sent via the Internet as opposed to the Cellular network.

If one or both are only connected to cellular networks the bubble colour remains standard green and are sent as standard SMS/MMS messages for which network charges apply.

Both use the same iOS messaging app !

Both iPhones need to have an icloud account iMessage enabled and be connected to a Wifi network.

One or the other or both could be connected to the cellular network for internet access and exchange iMessages. Wi-Fi access is not required for iMessage. Internet access is required no different from email.