Hi,
Thank you for using GWD.
It is easy to add the keyframes at the exact same time.
If you have a keyframe at one layer at 4sec mark, select the keyframe, which should move the playhead at 4sec mark.(The above layer has no keyframes yet.)
As the keyframe is still selected, hold down Shift key to Click to select the above layer, which will select two layers to gether. The playhead should still be at 4sec mark where the last keyframe on the bottom layer is. Then, hit F6 key (Fn+F6 for Mac). That will add keyframes to the playhead position to all the selected layers. In this case, the bottom layer already has a keyframe at 4 sec, it won't add another keyframe there, but it should add a keyframe to the above layer.
Could you try this and see if that works for you?
Regarding guides off the rulers, GWD is not currently supporting the feature, but we have a feature request filed internally.
Thanks,
Mariko (GWD team)