model <- function(t, state, parms) {
  with(as.list(c(state,parms)), {
    dN1 <- r1*N1*(1-N1/K)
    dN2 <- r2*N2*(1-N2/K)
    return(list(c(dN1, dN2)))  
  }) 
}  

p <- c(r1=1,r2=0.1,K=1)
s <- c(N1=1,N2=1)
run(after="state<-state+rnorm(1,mean=0,sd=0.2);if(state[1]<0)state[1]<-0")
run(after="parms[\"K\"]<-rnorm(1,mean=1,sd=0.2)")