We just butt the Coro together for jobs like this.
We buy it precut into 24 x 36, or 24 x 24.... Then it's always perfectly square. Butt them together on the flatbed, print them... Then cut them down to size after that.
Our supplier will cut to size for no charge since we buy so much Coro. You could probably even get them to cut them to 48 x 48.... And then you're just putting on 2 sheets.
It's a bit more labor.... But perfect alignment everytime. Half the time we order Coro one skid can be 48.5 x 96.5.... the next will be 48.4 x 96.2.... it's always oversized, but varies from skid to skid.... Not worth the hassle to deal with when doing multiple up, double sided.
Single sided isn't bad because you print on the left, bottom side.... Then use that as your square side. Cut everything 18" or however from that point and all
signs will be the same size.