If you're talking about normal lifesteal (not vampiric aura or -potion), it does never stack. The one which is used is determined by the following rule:
Take the ls ability from the items with the highest slot priority (if an item has multiple ls abilities take the last one in the list) and compare it to the last aquired non-item lifesteal ability. The one which is acquired later is used.
Damage block stacks to some extent. On one attack only one damage block ability is able to take effect, while the last acquired (pick up for item or lvl 1 for a normal skill) ability is checked first.
Example:
You pick up an item that blocks 50% 100 damage. After that you pick up an item that blocks 100% 1 damage.
-> you will always block 1 damage and never be able to take advantage of the better ability.
If you first pick up the 100% 1 damage block and after that the 50% 100 damage block, you will in 50% block 100 damage and in cases where the good ability fails block 1 damage
Mind that damage block follows the pseudorandom distribution Blizzard implemented and therefore has a bit lower chance to proc than actually specified in the ability.
(except at an chance of 100%, that one really is 100%)