Can the Senior Supplier and the Senior User role be provided in the Steering Committee by the same person? What are the risks and how they could be mitigated?
I believe that these roles CAN be combined, with care. I would expect that in such a circumstance you are delivering a project using resources and tools provided by one manager, where the project is intended to deliver something into his or her team. I had a similar situation where I was managing a project to deliver an upgraded set of file transfer capabilities to the file transfer team in a large financial organisation, where the manager of the file transfer team would be responsible for the new tools, and was the line manager of the technical staff who were allocated to my project. The manager was both senior supplier and senior user in this project.
The biggest risk (in fact it was an issue) that I found is that the manager would treat the resources as "his" property, and pull them off onto other work even though they were meant to be allocated full time to the project. He didn't have to justify such actions to the senior user because well, he was the senior user, and he could justify any consequent delays to himself. He also had a habit of switching resources around between the project team and his wider team, which made continuity very difficult. I discussed this with him on many occasions but he would not change his approach, and would defend it when challenged by the project sponsor.
The other risk - or rather an issue - was that he would change direction almost on a whim, which could have a significant impact on all sorts of things from team morale to financial performance to delivery dates. It didn't matter that we had a well-defined project with good controls, so I ended up spending a great deal of my time writing and submitting change documentation, which he, as Senior User AND Senior Supplier, would inevitably support, and use his position to pressurise the other members of the Steering Group to accept.
Having said all of that, he was a great person to work with: very appreciative of any progress, and generous with praise for a job well done. He knew what he was doing in his joint role, and was quite open about using his position to benefit his wider department, even if it sometimes impacted on the project.
Unfortunately, I never found a good way to mitigate these issues, apart from insisting that any change would only be put into effect following approval of the change request, and any re-alignment of resources would be reported at our regular weekly project meetings as an area of concern / risk / issue, as appropriate, which at least got it raised to the Exec and Sponsor.