Do Not Create Two Weak Groups
Weak groups are always a hassle. You will almost certainly have to use stones to defend them against attack and they normally end up with little territory related to the effort you've invested.
However, sometimes you must create a weak group in order to take a chunk out of your opponent's territory or to limit his expansions, and it may be a necessary evil.
Creating two weak groups will, in most cases, give you a larger burden than you can live with and the strength your opponent can build up whilst attacking them will often be too much to live with. Conversely, giving up or losing a group easily accounts for 20-30 points. Again, this is more than the average balanced game can take.
-- Morten Pahle
The reason why two weak groups are worse than simply 1+1 weak groups is that the opponent can play in a way, so that eventually an attacking move threatens both groups simultaneously. As you then can defend only one group, the other group might die, or the opponent gets a big chunk from it. These kinds of attacks are known as splitting attack or leaning attack.
-- Arno Hollosi
Question: What is meant by "1+1 weak groups"?
Answer: (Charles) If you believe QARTS or any similar counting method, you will allow a certain cost for settling a weak group. Now, if you have two weak groups, what is the cost? If you make one weak group, settle it, and then make another weak group and settle that, you can add the costs. But what is being said that if you make two weak groups at the same time, you can't add the costs. It is probably worse in terms of positional judgement; because that has to reflect the dangers.