What's the difference between SISO and SIMO in using matched Filters
I don't know why we can't use RRC filter with OFDM ?!! Yes, of course, we do can. The only disadvantage for that is the complexity. however the gain will be in spectral efficiency improvement and bandwidth efficiency because of avoiding the needs of guard interval which is used in the regular OFDM. In Engineering, we should always ask, why and why not? why should we use it and why can't we use it? 

So, Firstly, it's important to know that ISI in context of OFDM is different about ISI in the other types of modulations. As known,  the ISI happens when the symbol, in time domain, has a tail to the time of next symbol, but  that's not the same case for OFDM. In term of OFDM, that issue can be solves in two steps, because of the orthogonality, (and remember the use of FFT and iFFT), and second we insert the guard interval like CP which create circular convolution to eliminate the ISI. Be careful, in multi-path channels and large delay spread, that ISI will be exist in OFDM !! so we should present new solutions to deal with. "That exactly what the example reference,you provided above, tries to deal with that issue too". 

But the question you will ask now, Why regular OFDM doesn't use RRC filter?  In reality, As I mentioned above, Regular OFDM is using Rectangular filter instead of RRC filter for minimal complexity, but the cost of that low complexity is a.) inserting CP which reduce the bandwidth efficiency b.) spectral efficiency reduction by sidelobes. 

These two issues in regular OFDM, CP insertion and low spectral efficiency, can be solved by using RRC-OFDM, this RRC-OFDM doesn't use CP and the spectral efficiency is improved very well there. That technique was first presented in 2001, and then patented as US7065150 in 2006. 

Coming back to the question, Should we use one matched filter in received SIMO signal or Multi filters. That depends on the simulation you are doing. you can use only one, but be careful how implement it. Or try to filter each copy of signal separately and then collect them back again and continue your work.  

thnx