Estimating unknown parameters and initial values of state variables with the restriction that I want the initial state values of two of the state variables to be estimated to an equal amount

I am using a set of 5 non-linear differential equations (with 5 state variables):

dx1/dt=f(x1,...,x5)

dx2/dt=g(x1,...,x5)

dx3/dt=h(x1,...,x5)

dx4/dt=m(x1,...,x5)

dx5/dt=n(x1,...,x5); f,g,h,n and m have unknown parameters that need to be estimated

 I need to estimate unknown parameters and initial values of the states but I have a restriction: the initial values estimated from the first two state variables have to be equal(I need x1(0)=x2(0) after estimation).
I have tried creating an IDNLGREY object and using PEM and FINDSTATES 
for estimation of the parameters and initial states independently, but I do not know what to do if I want x1(0)=x2(0).

thanks