Combine them as in show them in the same axes with the same bins, combine them as in add the bin counts together, or something else?
If the first, call hold between your histogram calls. See the "Plot Multiple Histograms" example on the histogram documentation page. You may want to set each histogram object's BinEdges to be the same if the data isn't as closely aligned as you want, or you may want to select both histogram objects (click the arrow in the figure toolbar, shift-left-click each histogram) then open the context menu by right-clicking and select the "Align Bins" option.
If you want to add the bin counts together, I'd probably call histcounts for each of your data sets with the same BinEdges, then call histogram and pass in the common BinEdges option you used in your histcount calls and the combined bin counts as the BinCounts. This is the "histogram('BinEdges',edges,'BinCounts',counts)" syntax listed in the Creation section of that documentation page.