Yes I have several times over the years, though usually it was 1 large show (multi day), and one small show (single day). Sometimes my partner did the other show, mostly this was back in the day when he wasn't part of the business and not very good at selling or talking to people. Much improved now.
Other times I paid someone - a friend or acquaintance, though they never had to do set up or tear down, my partner or I always did that, or they would work the multi-day show (where everything was already set up), while I went to the one day. So it was never physically demanding for them. If it was a long day, we would arrange someone to come by just to give them at least a 15 minute break.
What did I pay? - minimum wage and sometimes a small jewellery gift of their choice (value $20-$30). I told them what I wanted to pay them, and asked if it was okay. Never had anyone say no to the rate, or want to discuss more, though I would of been open to a bit of discussion.
If it's worth it does depend:
a) on the shows - expected sales volumes, versus booth costs and salary costs.
b) amount of work required to split the stock, get the displays ready, make sure you have 2 of everything to run the booth (meaning your show kit)
c) how good the helper is at selling your goods, and how much training they need. Re: setting up display, or just being able to knowledgeable about your work and your process.
a) Was it worth it? In my case, the one day show was usually a bust, and not worth all the stress and effort of splitting the stock. Sometimes these were "new" shows, and sometimes these were decent shows that had a bad year. There was one town festival that had different vending opportunities - street vending with mix of vendors (crafts, commercial, charity), and indoor vending across town where you had to be approved to fit the theme, but the fee was cheap, $40. The first year it was well worth doing both, but the next two years the indoor booth sales were not worth all the time and effort of preparing two displays plus the cost of a helper.
However this summer 2 multi-day shows are overlapping, and we've booked them both and have to find a helper for each one, since each show is too busy for one person to handle all day.
b) Do you have enough stock? Do you have two displays? two showkits?
Who is going to be doing the setting up and tearing down at the show the helper is doing? Can they easily do it themselves? Do they have their own vehicle that it big enough for it all?
c) Helpers really vary in their skill level and how comfortable they are talking to people. I'm not a hard seller but I do greet people and I have "patter" where I talk about processes or designs, etc. So no one sells my goods better than me, though my partner has really improved over the years.
If your product totally sells itself (such as food), then it's easier to find someone to man your booth. But if more interaction and finesse is required, then it's important to find someone who has sales experience or who really knows your products.
There's a big difference between say leaving your booth to take a break while someone else runs it, and having someone else run it the entire event. I've had people with some sales or waitressing experience, people with little sales experience but who were sharp and learned fast, people who were shy, and 1 person who was practically useless work for me . Family helped too in the early days, my sister who was pretty good, my mother who is not good, and a cousin that was shy but okay to work along side with, but I wouldn't let her run my booth for more than an hour.
If you decide to do it my tip would be to plan early. Make a list. Start gathering goods for your second show kit. If you can, have the helper come work along side you at a different show at least for a few hours so that you can train them, and get them familiar with everything.
Having enough stock overall was never a problem for me, I usually display too much anyways. But the work of picking through and pre-pinning two displays has always been a hassle I don't enjoy. I'm always trying to "balance" as I pick and choose through stock, worried that I'll run low on one design at one show, while at the other show the pieces will just be sitting there.