a feature to explicitly allow multiple versions of a single dependency

[guest]

I wanted to build two executables for comparing the performance difference between happstack-server-0.3.4 and happstack-server-0.4. So I wrote below cabal file but found that cabal cannot build what I want. Here is the output for "cabal configure":

aycan@fgt:~/haskell/tests/happstack-tests$ cabal configure 
Resolving dependencies...
Configuring happstack-tests-0.1...
cabal: At least the following dependencies are missing:
happstack-server ==0.4 && ==0.3.4

Name: happstack-tests
Version: 0.1
Author: Aycan iRiCAN <aycan.irican@core.gen.tr>
Stability: Experimental
Category: Web
Synopsis: Tests for Happstack Packages
Cabal-version: >= 1.6
Build-Type: Simple

Flag base4
    Description: Choose the even newer, even smaller, split-up base package.

Executable hello-0.3.3
  Main-Is: Hello.hs
  Hs-source-dirs: happstack-server
  GHC-options: -threaded -Wall -Wwarn -O2 -fwarn-tabs -fwarn-unused-imports
  Build-depends: 
     base
   , happstack-server == 0.3.4

Executable hello-0.4
  Main-Is: Hello.hs
  Hs-source-dirs: happstack-server
  GHC-options: -threaded -Wall -Wwarn -O2 -fwarn-tabs -fwarn-unused-imports
  Build-depends: 
     base
   , happstack-server == 0.4