There are places in Ntcheu where the Malawi and Mozambique roads run parallel to each other with no border fence or any other barrier or markings. You can turn off onto the Mozambique road, fill up the tank and turn bank onto the Malawi road without anyone even noticing.You only realise it is Mozambique because the shop signs on the Western side of the street are in Portuguese while the Eastern side are in English.

Back in the days when Malawi was suffering from acute fuel shortages, the AXA bus service from Blantyre to Lilongwe used to actually stop by the side of the road near the border in Ntcheu and a herd of porters carrying jerricans of fuel would rush across from Mozambique to fill up the tanks.

Of course, if you want to go via the official checkpoints it is a fairly lengthy process with passport checks, insurance requirements, customs declarations, etc... Although my experience is that once they realise you are Malawian (or in my case, a Permanent Resident of Malawi) they are happy to wave you through quickly. Just be able to respond to the "Muli Bwanji" greeting with "Ndili bwino, khaya inu" and you will be fine - they assume that anyone who speaks even that much rudimentary Chichewa is an honorary Malawian.