"There are two different kinds of partnership – ordinary and extraordinary.

In an ordinary partnership, each member is liable in solidum (for the whole), or jointly and severally, for all the debts and obligations of the partnership.

With an extraordinary partner-ship, the liability of one or more of the members, often a “sleeping” partner, is limited to a greater or lesser degree, which falls for consideration under the Limited Partnerships Act 1907, or the Limited Liability Partnerships Act 2000."
http://www.estateagencynews.co.uk/reports/judicus_articles/j...

"Debtors are joint or several; joint, when they all equally owe the debt in solido; in this case if a suit should be necessary to recover the debt, all the debtors must be sued together or, when some are dead, the survivors must be sued, but each is bound for the whole debt, having a right to contribution from the others; they are several, when each promises severally to pay the whole debt; and obligations are generally binding on both or all debtors jointly and severally. When they are severally bound each may be sued separately, and on the payment of debt by one, the others will be bound to contribution, where all had participated in the money or property, which was the cause of the debt. "
http://legal-dictionary.thefreedictionary.com/Debtor

I don't think the use of "Monsieur" has any particular significance, certainly not in legal terms :-)

Since English is not my native language, I'm not sure of the legal wording, but you are right that it is either one or the other - or both but only to the amount indicated. The singular monsieur indicates this as well. In solidum: Au tout; caractérise l'obligation à débiteurs multiples conférant au créancier le droit d'exiger de l'un d'entre eux la totalité de sa créance (Lexique juridique Expressions latines, Henri ROLAND)