Hi,

When you breastfeed, you release the hormone Cholecystokinin (or CCK for short) Your baby also gets a burst of CCK while he is breastfeeding, which makes him go to sleep - he gets a longer more sustained burst as his food digests.
CCK makes you sleepy. You are in fact meant to get a rest. And breastfeeding helps that. As you recognise, having two young children is tiring. Breastfeeding assists you getting a rest. You are in fact meant (by nature) to curl up with yur baby and have a sleep. Encourage your toddler to lie down and have a sleep.
There is a myth that "breastfeeding takes it out of you" (Milk is the only thing it takes out of you!) In fact breastfeeding is very efficient. Breastfeeding mothers utilise nutrients from the food they eat more effectively. And they have savings in things such as iron, as menstruation is usually delayed. So in fact, they are generally in better shape than if they were formula feeding.
Breastfeeding 2 hoursly during the day is pretty normal and not "taking a toll". If you are eating normally there is no need in particular to take a vitamin.
Having a daytime nap with your babies is delicious. Its hot outside - the housework can wait. Enjoy it!