Note that ax1 and ax2 are the same axes:
ax1 = app.UIAxes;
% ...
ax2 = app.UIAxes;
You won't be able to have multiple colormaps in one axes, but you can make a second axes, invisible and overlaying the first.
Something like this would work:
x=randn(1000,1);
y=randn(1000,1);
f = uifigure('Position',[50 50 1000 500]);
ax1 = uiaxes(f,'Units','normalized','Position',[0.15 0.1 0.7 0.8],'Box','on');
ax2 = uiaxes(f,'Units','normalized','Position',[0.15 0.1 0.7 0.8],'Visible','off');
histogram2(ax1,x,y,linspace(-3,3,30),linspace(-3,3,30),'DisplayStyle','tile')
scatter(ax2,x,y,10,x.^2 + y.^2, 'filled')
linkprop([ax1,ax2],{'CLim','XLim','YLim','Position'});
ax1.XTick = [];
ax1.YTick = [];
colormap(ax1,'hot')
colormap(ax2,'spring')
cb1 = colorbar(ax1, "westoutside");
cb2 = colorbar(ax2, "eastoutside");
cb1.Label.String = 'histogram';
cb2.Label.String = 'scatter';
cb1.Label.FontSize = 12;
cb2.Label.FontSize = 12;
