Peeling, it is unacceptable IMO to leave a baby crying for 45 minutes when you could do something to stop it being upset, but you're choosing not to...
If you're leaving them to cry because you can't do something about it, because you're already doing something to look after the baby's twin, or an older child needs something more urgent...or even because you have no idea what it wants, have tried everything and are about to reach breaking point - that's very different.
Sometimes there are things you need to do that are more urgent than see to a baby (not masses, but there are some) sometimes it makes no blooming difference what you do anyway, lol.
Anyone with more than one child has to leave a baby crying at one time or another, with twins of course that's going to happen more, that's not a reflection on you - that's their fault for being awkward and arriving at the same time, lol