'...Photoshop doesn't have a tile printig + borderless option, it's either / or,...'
Actually, Photoshop has neither of those. Both are functions within the print driver.
You could try setting borderless and ensuring expansion is set to none (this is adjustable in an Epson driver, unfortunately I don't have a Canon printer here to check if that is available to you). However, that would still be subject to variance in the paper alignment between the two prints which is why borderless is often slightly expanded.
If you can't get hold of an A3 printer, you may need to make two seams and print three A3 sheets with overlap so you can trim the A4 sheets together.
Dave
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