Hold applies to axis, when having multiple axis I would always use the "long" version of hold (hold(axis_handle,'on')
)
figure(1);
hdl.r = subplot(121);
hdl.t = subplot(122);
hold(hdl.r,'on');
hold(hdl.t,'on');
plot(hdl.r, 1, 1, 'ro');
plot(hdl.r, 1, 2, 'ro');