Let us take an example if test case are written in MS Excel and is being updated parallel by couple of people and like to checkin end of the day, just the same case as you. Only tools are different.
In this case, I believe, it is not a problem with the tools(neither git, nor SoapUI), because they are working as designed, but only the way how we use it.
# At any point of time, each engineer has to look into the repository before checkin if there are any updates made(by someone) to the same file that he/she is working. If there are update for the same, then there is a procedure to follow.
Have a back of his changes(may be into a different file) before taking pull. To do this - Revert his changes, take update then merge his changes and then commit.
This is the general practise those who uses any repository for their projects. So, need to get acquainted with it.
If you think of a developer, each engineer may have to deal with this problem with multiple files each time which is more complex case than you. So, it is advised to make check-ins at the earliest possible.
Two people started working by taking the same git revision.
In your case, since there are only two people working on it, first person who commits his/her changes to git, will have an advantage as git accepts his file(as there were no updates after took pull), but the above # rule applies to him as well. And the next person have follow # .
There exists an another case where the first person will have to follow # on the following if starts working on that same file without previous updates of the second person from the previous day.
Just a thought. SoapUI also allows to export a particular test case(use right click on a test case) or particular test suite into an xml file. May be you can also take advantage of this and after having updates, just import only that suite or test case that he added for the day. This may just help in merging(without using git merge) little faster.
