In my answer to your last post, I commented that I've been there. Based on this one, I can tell you I've been there as well when my husband had a series of hospitalizations.
My guess is, your husband is feeling his mortality. That's normal. So are you - also normal. But it's the least likely time for teens or tweens or even many 20somethings to want to reach out. Seeing a parent as ill, vulnerable or in danger is horrible for kids, especially those who have not been raised to do the right thing or to know how to act/respond. In fact, if they are scared about their dad, they may be clinging even more closely to the other parent.
You have to get out of the middle of this, hard as it is. You telling them what is needed is going to come off as "you girls are bad daughters and I'm here to tell you how to fix it." I know you mean it as "I'm here to help and support my husband" but that's not how teens are going to see it. You are asking them to do something a lot of adults can't do - and they aren't your kids.
Your husband needs to either call/text them himself (and it should be "I'm going to get well so don't worry") or he needs to just let this go, focus on his recovery, and then call his girls when he gets out. And that's without the added drama of their mother, which is at play in this case. Still, you cannot guilt trip teens into calling - it will backfire.
I totally understand what he wants. I totally understand what you want. And sadly, from experience, I totally understand that the kids are not capable of doing what you want.
If it were me, I'd wait to get hubby home and then have him call the girls to say he can't wait to see them on the scheduled visitation weekend. If there's a problem with Mom bringing them to the drop-off point, can you two make the whole drive yourself? The easier you make this, the more likely it will be to happen. My husband did 99.5% of the driving for 12 years - the ex ONLY drove if a kid had a party and she could make a show of taking one OUT of our house during my husband's scheduled time. It takes too much energy to fight that. Don't try.