Now, previously, using the older systems, this would have been a serious pain in the ass. Much as I love 3.5, the challenge rating system was...rickety....at best, and an apparently balanced battle often ended up being way too easy or impossible (especially when it featured undead, who usually had a lot of hit dice, and as a result had attacks with totally bonkers save D.C.s).
Luckily, the more solid maths in 4e makes this sort of thing less daunting, and I have not become a weeping mass of gibbering whilst planning the next game. Having said that, I have had to bear in mind several important factors...
(I will use the expression "acceptable level of risk" quite a bit in the following. For me, this is a level of risk where, barring bad strategy and bad luck, the group have a good chance of surviving though not without some scrapes and item use. However, it also takes into account that a battle should be challenging, and that yep, characters can - and should - die from time to time)
Problem 1: One group (Group A) has all the Leaders, Healing and big hit point characters, whilst the other (Group B) has all the strikers and a single non-standard defender.
This is a biggie. I have chosen to set the battles Group A face about equal to their level or 1 less (though they can expect several of these in a row with little chance for a rest). I have been able to stick a wide variety of foes in these battles, as they have the means to tackle them with an acceptable level of risk. Group B will face varied opponents, but more of the artillery, minion and skirmisher types. I will also try to work some ways for that group to trigger healing surges without a Leader present - though what these are will remain a secret.
Having said that, Group B will face some more solid opponents, and out of the two groups, will face slighly roughter battles - such is the plight they are in. Most of their battles will be their level -1, but will feature multiple foes of their level -2, to -1. This should balance things out well - fast paced action Vs a massacre, and should (shoooould) keep an acceptable level of risk in play.
Problem 2: I have to do some railroading to keep it interesting.
Group A can, if they choose, bog off to the nearest village and wait to see if the Group B turn up - a common sense move, but not a very interesting one for Group A's players.
The solution. I have to railroad them a bit, to get them involved in some fighty-blasty-pain goodness. As a general rule I hate railroading, but in this instance, it will mean the difference between half the group sitting there doing nothing and half having all the fun, or everyone being involved.
Problem 3 - Two groups at once equals twice the effort right?
Wrong. I intend to run the whole session on an initiative count, with both groups and their foes being run simultaneously on two battle boards. To allow characters to hold actions without fear of being relegated to the later counts for the whole session, a Move Action will be allowed to let a character re-roll initiaitive, or a character can spend a whole round refocusing a la 3.5, to set their initiative as if they had rolled a "20".
Problem 4 - XP!
So, what happens when one group faces higher challenges and the other get less XP? They level up at different times and everything gets a bit complex. I actually got the players' opinions on this, making them aware that they would face slightly differrent levels of threat and so earn different amounts of XP. They all agreed that we would add all the XP up, and split it, 50-50 after the fact, which makes my life easier and means no one gets sniffy and pissed off!
And there you have it, the main problems I have faced and the cures. This next game will be real test, and will be the most challenging game I have run over the last 21 years. Check the blog out to see how it went (it's next Saturday).