Why work on one thing at a time?

Why work on one thing at a time according to the buddhism ? Did Lord Buddha advise us to do that ? Are there any deep desires in multi-tasking ? (Like wanting to gain more in short amount of time) **Edit:** I mean by "doing one thing at a time" is working on 2 different subjects at the same time(Without finishing the first work) . Ex: practising samatha and vipassana together, learning maths and arts at the same time. **Edit:** Example for "not doing one thing at a time" : Started to write a book yesterday. Whithout finishing it, starting to write an another book today.