Hrqls: is it always best to capture 2 pieces of your opponent ?

Not always but much more often than not. The stronger your board the better because the chances of their missing a turn or of only getting one man back in increases dramatically. With 3 points blocked, the table is 75% open for a single man on the bar but with two there the chances of getting both back in is only 25%.

suppose your opponent has 2 singles in his home, is it wise to hit both, but leaving 2 singles of my own in his home ? (again, early in the game)

I'm not sure I understand you here. Blots in his home are to be snapped up in great gulps!

What's a blitz?

This is a devastating attack, often starting with a double against split backrunners, in which the table gets rapidly closed. In the worst case (for the victim) they get to make one move at the start of the game and then have to wait until their opponenent is bearing off before they get another go.

An example.

Going for a blitz usually entails risk, eg. lots of blots as builders are brought into the area to get the table closed, midpoint blot left behind, backrunners left stranded. If the victim recovers (and again doubles often feature quite strongly), the attacker can be at a disadvantage and have the tables turned on them.