"partly so that" is grammatically correct and can be used in written English.
It is often used to indicate the reason or purpose behind a particular action or situation. Example: I decided to take up a new hobby, partly so that I could meet new people and partly so that I could learn a new skill.
America does indeed want flexibility partly so that it can run the show.
Partly so that visitors can admire prospective purchases, mirrors lurk everywhere.
The Southeys remained at Greta Hall for life, partly so that Sara and Edith could be together.
The largest American manufacturers have put much of their production closer to their global customers, partly so that their costs and sales are in the same currencies.
Some time ago, I booked a vacation for this week, partly so that I would have plenty of time to watch the climax of the World Cup.
The Internet Fraud Complaint Center serves as a clearinghouse for complaints partly so that it can glean patterns from the reports.
While many countries are pushing for a legally binding treaty, the Obama Administration is insisting on a sort of legal chimera — partly binding, partly not — so that, if there is a pact, it won't require Senate approval.
So that's partly our motivation.
If so, that would partly support Mr. Bush's contention that the Iraqis could have moved 380 tons of material very far without being detected.
Healey clearly sees his housing role as a key economic driver, stimulating a private and public housing development sector - partly funded by the taxpayer – so that action on the ground is visible over the next few months.
But one executive involved in the reorganization said Ms. Centrello did not begin her search early partly for reasons of discretion, so that word of the change would not seep out in advance.
