IPhone 4S iOS 5.0.1 Dual Sim failure

First of all I want to clarify what I mean by Dual Sim. I am using the iPhone with this SIM from the first 2G model, and it was working perfectly.
It is one sim with two phone numbers (one forwards the calls to the other). This options is offered by the Romanian carrier Vodafone for business clients.
I am not talking about two phisical sim cards.
Starting yesterday evening after upgrading to iOS 5.0.1 on my iPhone 4S (bought in Germany) I can't really use the phone anymore, I hardly have any signal and nobody can call me on both numbers.
I changed the sim with a new one from Vodafone but had no luck.
The sim is working anyway perfectly on other iPhone models (3g, 3gs, 4) with a lower iOS version than 5.0.1.
I really hope Apple will issue an update as soon as possible because right now I have to go back to an older model in order to use my phone.
Any help and/or feedback would be appreciated.
Thank you,

Hi Sebastian,
I do have a similar problem. I also use a dual sim that works on iPhone4 and iPhone 3GS perfectly. I obtain the dual SIM service from the Spanish carrier Movistar.
If I go to Settings > General > Phone > SIM Applications > Dual, I am able to see wich line is active, but I can not change from one to another. If I try to change the line, the phone loses its network service and I have to switch it off and on again to recover the network service and being able to talk, text and navigate.
With 3GS and 4 I could perfectly change from one number to the other one (Before I had a Nokia N97 and it also worked perfectly). With my brand new 4S (iOS 5.0) I do have to switch the phone off and on again in order to change to the line I want to use.
Movistar Spain has recognized there is an issue with iPhone 4S and dual microsim cards. They say  they are waiting for Apple to approve some kind of new version (I guess they talk about a new iOS version but I'm not sure).
I know there is more people at Spain using this service with Movistar and they are becoming really angry (using 4S with iOS 5.0 and 5.0.1), because this issue was reported more than a month ago. It's really frustrating having paid for such an expensive phone and suffering this kind of problems...
I hope Apple will give us a solution really soon.