Tendering: Whole Job or Shell Only then Fitout seperately ?

I have been told to tender for the Shell Only, then run a separate tender for the Fitout (eg kitchens, bathrooms, coverings etc). Apparently, this limits the markup builders will make on Prime Cost items, Provional sums and Variations as the Shell has more certain costs associated as well as giving you more time to firm up the interior design and fitout (where most of the variation is). Other feedback is that it makes the builder price more keenly for the second part of the job.

What are your thoughts here ? Any experiences shared would be much appreciated.

Granted, it would make the job longer but we would live through the reno so rent not an issue (sanity however....)