It *IS* difficult to train with only one, when you have two!

If Maggie does have the basic concept of "Come" down pretty well -
you could "try" drilling them together in the yard.

Run it like a contest - Furst Dawg that "Comes to Momma" gets
the GOOD Treat;
#2 gets the Not-so-Good Treat (or maybe just a pat on the head).
As long as they BOTH respond to the "come" and remain focused
on the drill - a little competition may sharpen their recalls.

But if one "quits early" and doesn't "come" all the way to you -
I'd resume one-on-one practice.

When SmokeMutt was *In Skewel* and we were doing drills, I did
sometimes have to "tie" Cinder up or leave her inside while
I gave 100% Attention to Smokey...
But as he became more skilled we did a lot of Dual-Dog drills
where they were clipped to the same leash with a very short "Y-Thingy"
that kept them shoulder-to-shoulder. I'd just alternate which
one was my Leg-dog and which was the Outside-dog.

They can and do perform all general commands like HEEL, SIT, DOWN,
and STAY while they're clipped together and rubbing shoulders.

/s/ Phred
.