it would be best to go 40min straight, but if you honestly can t do that then I dont see the harm in taking a quick break (a few seconds max and no getting off the machhine!!!) or doing 20 min in the morning and 20 at night. This is for you re health I m assuming, so as long as you get in the required minutes you ll be fine. If it were for weight loss on the other hand it would have to be all 40min consecutively to be effective.
Don t kill your self, try and walk if the weather is nicer. walk a block or two then walk a little faster for two blocks. Keep switching your pace. you will be able to go longer than 40 minutes and your not looking at the tv treadmill. You may meet someone and get some fresh air at the same time.
Doing 20mins in the morning and 20mins at night is just as effective as doing it all at the same time.Or if you lose your breath maybe just do it at a slower pace and do it for an extra 10mins or something.