As long as you have enough space (and the initial size is really pretty small), there is no problem installing the MDF and LDF on the same disk. In fact, many of our demo systems for small tests have everything installed on one disk.
The only other condition I can think of that cause SQL Server to prevent you from using one disk for more than one file is if you were using RAW instead of a formatted file system. When using RAW partitions, since there is no file system, there can only be one "file" per partition.
