I actually had a similar situation some years ago. I don't think my experience is very useful, but I will briefly recap it in any case.
I submitted a paper to a journal in October 2009. In April 2010, after several requests, I was told that one referee had given a report, and the journal was still waiting on the second report. The journal then sent me the report at that time (in April), though I gather that was not standard procedure, and as far as I can see, I did not explicitly ask for it, though I had been repeatedly asking for feedback of any kind. The journal wrote:
In fact, we got a report from the first referee. The referee's opinion
is rather negative. As a rule, we do not send a negative report before
we get a report from the second referee. Taking into account your
will, we send the first referee report to you - please find it below.
SInce we have not yet got the second referee report,
there is no editorial decision concerning your paper.
We hope to receive the second referee report in April.
I don't know exactly what was meant by "taking into account your will".
The second report never arrived, and I withdrew my paper in September 2010, slightly less than a year after submitting it.
I suggest that you ask your journal for the one report. You might want to wait on withdrawing it till you hear what the report says. You could say that you would like to use the first report to improve the paper while waiting for the second.
If they refuse, I guess you could then say that you are withdrawing the paper, and then ask for the report again. If you are going to withdraw the paper, this marks the end of the process from the journal's point of view, and I don't see why they would refuse feedback that might help you. Of course, they might refuse regardless. I imagine that individual journals would have different policies about such things.