Following Lang and Trotter we describe a probabilistic model that predicts the distribution of primes p with given Frobenius traces at p for two fixed elliptic curves over Q. In addition, we propose explicit Euler product representations for the constant in the predicted asymptotic formula and describe in detail the universal component of this constant. A new feature is that in some cases the ℓ-adic limits determining the ℓ-factors of the universal constant, unlike the Lang-Trotter conjecture for a single elliptic curve, do not stabilize. We also prove the conjecture on average over a family of elliptic curves, which extends the main results of [11] and [1], following the work of David, Koukoulopoulos, and Smith [6].Here we use the notion → as introduced in [5, Section 2.3]. More precisely, for a sequence (s n ), we set lim m →∞ s m := lim n→∞ s m n with m n := ℓ≤n ℓ n .We note that a similar conjecture has also been proposed for the case that E has complex multiplication (CM) and t 0 (see [2, Conjecture 1]). However, the analysis of the constant is different in the CM and non-CM cases. In this paper, we are only interested in the non-CM case.The constant c E,t can be zero in certain cases. For example for E : y 2 = (x − 1)(x − 2)(x − 3) and p > 2, one can show that a p (E) is even (see [17, p. 420]). Thus, by considering a p (E) as the trace of the Frobenius at p in the division field extension Q(E[m])/Q and applying the Chebotarev density theorem we conclude that |G E (m) t | = 0, for odd t, as m →∞. Therefore, c E,t = 0 for odd t.Lang and Trotter expressed the constant c E,t as a product of a non-negative rational number r E,t , depending on E and t, and a positive universal constant c t , depending only on t. Moreover, in [18, Theorem 4.2], they provide explicit expressions for r E,t and c t . A celebrated theorem of Serre [22] states that, for a non-CM elliptic curve E, the image of ρ E is open in ℓ GL 2 (Z ℓ ). Therefore there exists a positive integer m such that ρ E (Gal(Q/Q)) = φ −1 m (G E (m)). Let m E be the least such m. Then for m = m 1 m 2 with m 1 | m ∞ E (i.e. the prime divisors of m 1 are among the prime divisors of m E ) and (m 2 , m E ) = 1, we obtainUsing this fact, we may then write c E,t = r E,t · c t , whereThe Lang-Trotter Conjecture has been studied extensively in the literature. The best known unconditional upper bound for π E,t (x) for t = 0 is x 3 4 , obtained by Elkies [8] and Ram Murty, and is x(log log x) 2 /(log x) 2 for t 0 (see [20, Theorem 5.1] and [25, Theorem 1.4]). Under GRH, Zywina [26] has recently obtained an upper bound for π E,t (x) of size x 4 5 /(log x) 3 5 for t 0, and an upper bound of size x 3 4 /(log x) 1 2 for t = 0. The Lang-Trotter Conjecture was first shown to hold on average over a family of elliptic curves in the case t = 0 by Fouvry and Ram Murty [10]. This result was then extended to the case of non-zero integers by David and Pappalardi [7].In [18, Remark 2, p. 37] it is mentioned that, by employing a probabilistic model, one can state an analogous conject...