Your original question is quite a 'classical' cost/value/lifetime question: how do I solve a process requirement best? With one or several mechanical components?
The variables are on a general basis:
a) Process specifications (as stated by others above) eg. here: fluid specification, amounts, delta P's, pipeline lengths, surroundings, etc.
b) Requirements for components: (as stated by others) such as: quality, materials, lifelength expectancy, power specification and consumption, type etc.
The third, and often underestimated, set of variables is:
c) Commercial surroundings: price of components, availability in market, state of 'art' regarding development of available solutions (available pump technology: is this the best/latest and to what risk/cost), experience and references for service and suppliers, cost/lifetime as comparison to buying cost, risk and responsibility evaluation for different mechanical solutions.
Conclusion: Your question can be answered, but only at asking suppliers directly and this only if points a) and b) are specified and c) is caretaken and specified. That is: for one set of specifications in one market, at a given time, but not generally answerable for all markets at any time.
In addition: different pump makers might have different increasing price steps for differing sizes and types.
Advice: Could possibly suppliers be asked for preliminary budget prices and suggestions (in their opinion) for best alternatives, giving their comments on one or two step solutions?
If equal dependability of all components a two step serial
operation will generally tend to be higher in total installation, operational and maintenance cost than a one step operation, but this is not an 'absolute' truth.