Such an affidavit is expressed in the plural, such as "We, Jane Doe, rocket scientist of Houston, Texas, and John Doe, janitor also of Houston, Texas, swear severally as follows:" or, preferably, in the first person singular.
In countless Canadian and other decisions abroad, joint affidavits have been received and considered by the courts without any thought as to whether an affidavit made by more than one affiant at the same time and as to the same allegations is good evidence. In the Canadian Civil Procedure Encyclopedia, the authors devote a section to the topic without mention of inadmissibility.
But in some discrete jurisdictions, the form of the rules of court may preempt affidavits sworn to, together, more than one person. For example, in Elhatton v. Canada, Justice Rennie of the Federal Court wrote:
"Joint affidavits are unknown to our legal system. There are many good reasons for this; they inherently reflect a collusion between two separate and distinct witnesses and interfere with the truth-seeking function of cross-examination."
This, though, may be unique to the Federal Court of Canada as the 2013 Federal Court Rules, at §80(1) stipulate that:
"Affidavits shall be drawn in the first person...."
As for "joint affidavits are unknown to our legal system" at large, it is difficult to agree with that proposition of law absent a clear provision in applicable rules of court to the contrary. Joint affidavits are rife in the common law legal system. One specific area is within family law, where separating spouses seek to file a joint application for a divorce. As part of the evidentiary basis for such an application, the parties may agree as to the content of the material allegations and submit a joint affidavit in that regards - something they can hardly do "in the first person" unless that includes writing in the first person severally as opposed to first person singular.1
Joint affidavits are also very useful in estate proceedings when members of a family, for example, seek to repeat the same allegations of fact in multiple affidavits but instead, as their evidence is identical, do so by way of a joint affidavit. For example, a joint affidavit by the witnesses to a will attesting to the fact of the signature of a testator upon his will or, where holograph wills are allowed, attesting as to the hand-writing.2
Any search for the words "joint affidavit" in electronic databases of case law of any common law jurisdiction (e.g. the English Reports) readily returns many other examples of joint affidavits being received by the courts and considered without comment as to their admissibility at-large.
- Elhatton v. Canada (Attorney General), 2013 FC 71. See, also, Antoine v. Sioux Valley Dakota Nation, 2008 FC 794 at ¶33.
- NOTE 1: Brenner, Donald, Chief Justice, BCSC, Family Law Practice Direction #3, Oct. 18, 2002 at B.5. Also Family Law Deskbook, Desk Order Divorce (Vancouver: Continuing Legal Education Society of British Columbia, 2010), p. 8/10, ¶12.8. And this, in spite of rules of court that require affidavits to be "expressed in the first person". The 1961 BCSC Rules of Court specifically mentioned "affidavits made by two or more deponents". In British Columbia Court Forms 2012 (Irvine, F. and others, Toronto: LexisNexis, 2012, Volume 1, page 3-18 to 3-19), the authors ambitiously suggest that joint affidavits are inadmissible.
- NOTE 2: for example, In The Goods of Meade, Deceased,  New Irish Jurist Reports 7 and also In Re Davis, 163 ER 889 (1843)
- Stevenson, W., and Cote, J., Civil Procedure Encyclopedia, Vol. 3, pages 44-9. (Edmonton: Juriliber Publishers, 2003)