This weekend when we were riding, apparently we rode into some sand, which didn't look deep at all.
We were going approximately 10 mph and we developed a very big wobble on the front end.
We were luckily able to ride through it and finally get the bike stopped without crashing by riding it out and finally when my husband thought it was going slow enough, he put a foot down, as did I.

Is there any known method of getting the bike to stop the wobble?

I have heard of a high speed wobble, but never a low speed wobble.

The slow speed going through the sand could have caused the bike handling issue.
Did wobble go away after you go out of the sand? If it did, then that was the cause.
If not, check your headset.

As Rudy (aka, Zona) notes, if the wobble went away after the sand incident, then there's a pretty good chance that it was an isolated incident caused by your encounter with the sand.

I can easily imagine a scenario where the front wheel hit the sand that caused a direction change of the front wheel / steering input that your captain had to counter. It's quite possible that the quickness of the steering change and countersteer reaction by your captain caused both of you to begin shifting your weight off center out of sync with each successive turn of the handlebars thereby sustaining the "wobble", if you will, until he brought the bike to a stop.

I don't think we were out of the sand entirely when my husband got the bike stopped. Hope it doesn't happen again, that scared me as much as our first ride down the street on the new tandem

We had a similar incident a few weeks ago while at Long Point Provincial Park in Ontario. We were looking for a cmp site cloer to the beach and the road "looked" OK but we went down into the sand and we dumped. Luckily we landed on soft sand!