Are multiple Cookie headers allowed in an HTTP request?

Question

Usually, a browser groups cookies into a single Cookie header, such as:

Cookie: a=1; b=2

Does the standard allow to send these

Answer 1

it's now allowed in HTTP/2 (RFC 7540), which specifies:

    8.1.2.5.  Compressing the Cookie Header Field

   The Cookie header field [COOKIE] uses a semi-colon (";") to delimit
   cookie-pairs (or "crumbs").  This header field doesn't follow the
   list construction rules in HTTP (see [RFC7230], Section 3.2.2), which
   prevents cookie-pairs from being separated into different name-value
   pairs.  This can significantly reduce compression efficiency as
   individual cookie-pairs are updated.

   To allow for better compression efficiency, the Cookie header field
   MAY be split into separate header fields, each with one or more
   cookie-pairs.  If there are multiple Cookie header fields after
   decompression, these MUST be concatenated into a single octet string
   using the two-octet delimiter of 0x3B, 0x20 (the ASCII string "; ")
   before being passed into a non-HTTP/2 context, such as an HTTP/1.1
   connection, or a generic HTTP server application.

   Therefore, the following two lists of Cookie header fields are
   semantically equivalent.

     cookie: a=b; c=d; e=f

     cookie: a=b
     cookie: c=d
     cookie: e=f

Answer 2

Chanced upon this page while looking for details on the topic. A quote from HTTP State Management Mechanism, RFC 6265 ought to make things clearer:

5.4. The Cookie Header

When the user agent generates an HTTP request, the user agent MUST NOT attach more than one Cookie header field.

It looks like the use of multiple Cookie headers is, in fact, prohibited!