Do you have the same percentage of growth assets (ie shares/stocks) as your wife and over what time period have you measured the performance?

I realise I am not answering your question, but a deeper look into your fund may change your mind about moving.

In theory your KA provider is already spreading your risk.

That said ... my partner and I are with different providers as an additional risk spread.  I don't see any harm in doing so.  There are mutiple high performing funds.

Depending on your circumstances it could be worth having a savings scheme other than KS.  The major advantage of KS is employer contributions.  Above and beyond the employer contributions there is no disadvantage to investing elsewhere.  My employer only matches my contributions to whatever the legal minimum % is so I put additional funds into another scheme.

There are resources for comparing kiwi saver funds, eg sorted.co.nz

Don't forget the fees. 

I'm with Simplicity Kiwisaver a New Zealand run non profit scheme with decent returns and the lowest fees.

It does not have five year performance data in the comparisions as it has seems to have established June 2016. 

It was more the risk of having both funds in the same provider, rather than the risk profile of the funds, that I was wondering about.

I like the idea of being with two different Kiwisaver providers. Kiwisaver isn't government backed (nor are our banks), so it does spread your risk further against one provider failing.