I’m afraid at present you can’t specify that the dashed subdivision between the two time signatures in the aggregate time signature should not draw. I’ve been looking into this today and I believe we will be able to support this via the popover in a future version (though possibly not the next update, 1.0.30). You will be able to type e.g. 5/4|1/8 to get the result you get now, i.e. with the dashed subdivision, but typing e.g. 5/4:1/8, i.e. using a colon as a separator instead of a pipe, will produce an aggregate time signature with no dashed subdivision.