It is not necessary to keep fast of Shawwal for six days regularly, one can keep them at random:
وقد روي عن ابن المبارک أنہ قال: إن صام ستۃ أیام من شوال متفرقا فھو جائز (ترمذي)
So, if you want to fast on Monday and Thursday separately, you can do so. And thus, you will fast the remaining two days in order to complete the number of six. However, if you regularly fast on Monday and Thursday and while fasting the six days of Shawwal from 2-7 if you do not find time to fast the two days separately, there is no matter whatsoever, rather you can gain its virtue only by intention and regular habit in-sha-Allah.
Allah (Subhana Wa Ta’ala) knows Best
Darul Uloom Deoband