Pay for 2 Baby-sitters

I tend to be generous with babysitters bc in the grand scheme, a little extra usually means a lot to them while it's usually not a big swing factor in how much the overall outing cost me. Typically... (We don't go out much...) So if you like this sitter and will want her back and to do a good job, I think you're right to not want to be cheap. I'm amazed a jr high girl can handle that many when they are so young... I think slightly less than typical for each is plenty generous though. I'd almost say increase your hourly rate by 50% -75% and give it all to your regular sitter and let her sort it out between them. I think the 50% vs 75% is how much the friend is benefitting you vs the sitter. Does it give you a lot of piece of mind that there will be two of them? If so, then more towards 75%. If you think she'd be tired after the 10 hours but your kids would be happy and fine and the friend is really mainly for your sitter's benefit and you couldn't care less if the friend came or not, I'd go more towards the 50%. I always take into account how much the sitter is kind of doing me a favor vs happy to have the work...

ETA: people (meaning you Marda LOL!) notice she said they are junior high age, so 12-14 :-)
I would pay your regular sitter the usual amount (good reliable sitters are hard to find and should be kept well paid and happy IMO!) and pay the extra girl a fixed "bonus" amount, like $20/25 or so (?) and thank her for her help and support.
Make sure your regular girl knows ahead of time SHE is still in charge, and that her friend is just there to help.

You really need to ask your sitter. I wouldn't pay for 2 sitters at one time, I'd hire someone who could handle all of my kdis and pay my usual rate. I'm sure your sitter doesn't want to make less, but I understand you wanting to not pay double for childcare.

How old are the sitters; teens or adults? What is your usual rate? Ten hours is a very long time. Three children/babies those ages are more difficult to care for than older children. Any time over 8 hours in a regular job earns extra money.

All things to consider.

Sorry, I missed the junior high description. I would not leave 3 children that
age for 10 hours with one junior high student. Many young adults wouldn't be able to handle them in a quality way for 10 hours. I would give them at least
1 1/2 X your usual rate to share. And I'd tip because it was 10 hours.

I would pay your sitter what you normally do and give her friend a tip (maybe around $25) not sure what her regular rate is though. I live outside of Phila. and my 15 yo daughter makes over $10 / hour. The family she babysits for is very generous and always rounds up. (She babysits for a 6 yo child).

I would assume that the friend is doing an equal amount of work, so paying the main sitter her usual rate (say it was $10/hour) and then giving a bonus of only $20-25 to the friend (as others suggested) seems very unfair to me.

Since it is such a long day, you need to give them a fair rate to make it worth the exhaustion for them. I wouldn't pay any less than a couple of dollars under my normal rate (so, cut to $8 if $10 is normal) and give each girl the same amount (so, it would be $16/hour).

Another option would be to give all of the money to the usual sitter and tell her to split it with her friend. Leave it up to her to decide if she feels she should split it evenly or not. If you do that, I'd probably give 50-75% more than her usual rate.