First time yeast starter from washed yeast question.

I'm looking to save some money while stepping up to 10 gallon all-grain batches. My washed yeast looks great (just like I would get in a vial), but I have never done a starter before. I would love any tips and was wondering if I can just put the DME/boiled & cooled water into the mason jars that the yeastcakes are in, provided the lids are loose. I'll be dividing a 10 gallon boil into 2 6.5 primaries, so should I just do two separate starters or split one? Does it matter?
My primary expertise is in hop cultivation and usage, so any help here would be awesome. My beer budget is shared with my fishing budget, so saving on yeast at double batches seems worth the investigation. Thanks in advance and please forgive my ignorance.

The vessel use for the starter is not that important provided it meets sanitary requirements. So yes you could just pour off the water from the mason jars and add wort to them to make starters.

What is important is that you pitch the recommended number of viable cells to your wort when making the brew. Programs like Yeastcalc and Mr Malty will help you immensely to get that right.

The fact you are splitting the wort into 2 primaries is not an issue and you can either divide one large starter or do separate ones. Provided you end up with the correct quantity of viable yeast in each primary you are all fine.

Ideally if you intend to do large 10 gallon batches then I think you need to sell a fishing rod and use the money you get to make a stir plate and buy an Erlenmeyer flask.