I think the major difference between Kate and Meghan in context of separate offices is that Kate accepted all the royal family stuff as the price she pays for marrying william. She has a spine of steel, we all agree, but my sense is that in her official capacity she's happy to more within the established lines. Clearly she has things she wants to focus on and I remain convinced she was a big driving force behind the Heads Together campaign but she does't make waves. I have absolutely no doubt however that she is absolutely rigid on what she thinks is best for her family and that if there have ever been run ins with official flunkies its in that context - whether that be school choices, how much time she spends on official events etc.
Meghan on the other hand, found part of Harry's attraction the platform he brings with him. This does not make her greedy and shallow, but the point is that something that was very important to her was also important to Harry. I think we all saw a shift in Harry after he launched Invictus - it was like he woke up one day and realised that instead of railing against the unfairness of the fishbowl he lived in, he could use it to drive his OWN agenda. And this is something Meghan wants to do too.
But all this means that of course they need separate offices. They have completely separate roles and objectives which will diverge even more in time. Increasingly, while much of Kate and William's work will still have a charitable effect, it's gong to be about statewide issues. And with such a charming and personable couple the British government would be crazy not to use them for more of this. They'll be opening railway lines and shopping centres, turning up at government sponsored events etc.
harry and Meghan on the other hand will be doing more and more that have a charitable and/or societal focus. They will be less constrained, eventually, by their closeness to the throne. Thy will have FAR more room to drive the actual direction of their work and be able to choose what they spend their time on. They need a completely different approach.
And their new comms person is a political expert.... I think we can see Meghan TALKING even more, formally and informally.