One entry, and I would still give it the whole day. A city that has arranged its best hour around a garden.
One entry does not mean half a day. It means an unhurried one.
Go at opening and give the founder's rooms an hour before you allow yourself the garden. In the other order the collection never quite gets your full attention back, and it deserves it.
The garden is not an interval between two buildings, whatever the plan of the site suggests. Sit in it. It is the only museum garden I know that improves the rooms on either side of it.
February keeps the crowds down and the light long, and Lisbon in winter is a courteous city — which is not always the word for it in August. Check the closing day before you book; I have been caught out elsewhere for assuming.