A single wide mobile home can provide excellent shelter, however, they are relatively small. You may be tempted to increase the size of your structure, especially if you have two or more people living in a single wide unit. It may be possible to effectively turn a single wide into a double wide, but there are a number of important considerations and several pitfalls to avoid.
Most states, counties and municipalities have adopted some form of codified rules for building, often based on the national building code. Chances are, the codes with jurisdiction of your property don't just relate to new, stick-frame construction, but also to mobile homes and manufactured homes. The rules likely distinguish a manufactured home as being real estate and a mobile home as being a different kind of non-real estate property, known as chattel. You're well-advised to note this distinction. Regardless, any underlying foundation and utilities serving a single wide will be governed by building codes and will probably require a building permit.
Setbacks are parts of building codes that regulate how far structures must be from property lines. Most jurisdictions also have setback requirements between structures. This is important to understand because an additional, detached single wide structure will probably be treated very differently, than an additional attached structure. The detached structure, if allowed at all, will probably have to be separated by a minimum distance (often 10 feet) from the existing structure. If it is within the setback requirements, it will be probably be considered an addition and must comply with all rules of an addition. Mechanically connecting two single wides while remaining compliant with local building codes is unlikely.
A Second Single Wide Unit
Check with your local building department to find out if a secondary structure is permissible. Understand that building officials may not volunteer details for you to achieve this objective, they are obliged to give you factual answers. It is incumbent upon you to know what questions to ask, so be prepared to ask a variety of questions. You may ask if they would allow a second unit if an engineer approves the method of joining the two structures. You may also ask if you can decommission either the kitchen or bathroom in a second unit, install it, observing the required distance between buildings, then build a covered breezeway between the two structures, effectively creating a hallway.
Single wide and double wide structures are both built in a manufacturing environment; they are not custom building systems, so any changes to the building method would have to be specifically approved by an engineer, which is expensive. You may, however, be able to build a stick-framed addition onto your single wide — particularly if the single wide is sitting on a permanent foundation. Some codes may require code-compliance of an entire structure being added to, so if you build an addition, the rest of the house must also be to code. It's quite possible that building departments won't know how to handle a request to conjoin a single wide unit with an up-to-code, stick-built addition, but they may grant you a permit, especially if the original structure has been there a long time.