Hi Sara!
1. I am currently expressing once in the morning and once in the evening off both sides into the same bottle (is this ok mixing the two sides?
Yes, it's perfectly fine to combine the milk into one bottle or bag (in the instance that you're storing it in the fridge/freezer for later).
2. I only manage to express for about 5-10min on each side at the very most before hardly anything comes out but this amounts to roughly 4oz or so. ( will it increase)
Sometimes, it can be hard to get a letdown with pumping...a machine is a lot different than a baby. I always found it helpful to make sure I was really relaxed--take a deep breath, do some shoulder rolling, clear my mind, think about my baby, etc. What kind of pump are you using, or are you hand expressing? That can make a difference as to how much you're able to get. I used a double pump for 15-20 minutes at a time, just to make sure I was getting hindmilk...sometimes less if it was later in the day and my supply was low. There are things you can try to do to increase how much you pump, but it's also not uncommon for babies to actually be getting more while nursing than what we get while pumping.
3. If my DS wants more then 4oz can I mix the milk from two different bags?
If both bags of milk are good, then, yes...that's okay.
4. Most importantly though how do I know if I am getting to the hind milk and if my DS is getting that frn the bottle?
As long as you're pumping the hindmilk and you're mixing the milk up well after it's been refrigerated, your DS WILL be getting it when fed through the bottle. Hindmilk tends to look more creamy and when you let bm sit at room temperature for a little bit or when the bm has been in the fridge, it's that creamy yellowy white layer at the top. To help make sure you're expressing hindmilk, the length of time you do it matters. Well, there's no exact amount of time to equal when hindmilk is being pumped, but the longer you go, the more hindmilk there is. You can also tell when you're pumping hindmilk by the color I mentioned.
HTH a bit...I had to pump a lot for when I went back to work; so if you have any more questions, I'm happy to answer them.