Do you have to hold the portal for a set period of time to make the trade? Or can you trade as soon as you reach the portal? Also, if there is an active trade, can it only be made by one player? Or can multiple players make that trade? If the latter, how long does the trade last? Thanks!

You can trade with the other moon as soon as you get to the portal zone. The trade is an established process. Every night at 0:00 server time (it may be a bit later when the daily result processing takes some time) the dwellers of another moon are transporting their goods to the portal and waiting if any of the Beastlords will accept their trade. They have only one item per day, so they give it only to the Beastlord who offers to buy it first. The trade itself doesn't take any time as both sides are meeting at the same place with the goods and just exchange them.

Ok, thank you!