If there is overlap, then it is fine. Some people do multiple jobs at the same time. It's better to disclose. Just make sure your tax papers show the same.
Another example is for addresses. For example, if you live in one address is 2 years, say March 2013 to March 2015. The second address is April 2015 to present. The way you write the first address is 2013/03 to 2015/03. The second address is 2015/03 to PRESENT.
There has to be absolutely NO GAPS in your timeline. Thankfully we only have to mention Month and year and not the actual dates.
Good luck with your application